Walmart and Amazon have been fighting an e-commerce war, and now their efforts have turned to third-party merchants.

Walmart has a much newer third-party marketplace when compared to Amazon, but the retail giant is making a serious push to sign more sellers. To that end, Walmart linked up with marketing firm ChannelAdvisor to make it easier for merchants to sell on Walmart.com, according to Internet Retailer.

The company had six third-party merchants (including Wayfair, eBags, and ProTeam) when the marketplace debuted in 2014, but it now has approximately 300.

Walmart still lags behind Amazon significantly in e-commerce, but it does have the fastest growing marketplace, according to ChannelAdvisor CEO David Spitz, at 65% year-over-year in the fourth quarter of 2015.

All third-party items sold on Walmart.com are displayed among the company's own inventory, but they are labeled with "Walmart Marketplace" to differentiate them. The biggest incentive for third-party retailers is the ability to participate in Walmart.com promotions, such as Value of the Day.

Amazon, on the other hand, continues to grow at an impressive pace with regard to the number of products in its marketplace, according to RW Baird research cited by eCommerce Bytes. The number of products on the U.S. marketplace surged in Q1 2016 thanks to third-party retailers.

Furthermore, Amazon is still the leader in e-commerce traffic in the U.S., but Walmart's traffic is accelerating, according to comScore. Walmart had 84 million U.S. online visitors in June 2015, a 14% YoY increase. This was almost double Amazon's growth of 7.8%. But Amazon still eclipses Walmart with 180 million total U.S. online visitors in that month.

The battle between Amazon and Walmart will rage on as the latter tries to unseat the former as the king of e-commerce. And to do that, they will need to employ the best strategies to reach consumers and attract them to their marketplaces.

Within digital, consumers are spreading out their retail purchasing across channels, forcing retailers to spread out their online marketing budgets. Paid search, affiliate marketing, and email all increased their share of e-commerce referrals last year, according to Custora.

Paid search especially stood out as a major source of spending by retailers. Search ad spending grew 18% YoY in Q4 2015, according to IgnitionOne.

Mobile continues to drive the most sales growth for retailers, but sales still aren't keeping up with retail traffic. IBM found that smartphone traffic beat both tablet and desktop, making up 53% of all online traffic. But mobile still only accounted for 29% of all online sales.

Retailers only have themselves to blame for underperformance on mobile, as many still aren't using best practices for mobile websites and apps. Only 60% of the top 100 global retailers currently have a dedicated mobile website, according to The Search Agency.

The increase in online shopping has put stress on the shipping and logistics industry. The number of UPS ground packages delivered on time during the holidays fell from 97% in 2014 to 91% in 2015, according to ShipMatrix.

Retailers are beginning to explore alternative shipping options. Earlier this year Gilt Groupe switched its primary ground shipper from UPS to Newgistics.

Retailers that can't afford to invest in alternative shipping options are offering consumers more fulfillment options using what many of them do have — brick-and-mortar stores. Buying online and picking up in-store, also called click and collect, made up about 30% of e-commerce sales at Sam's Club in 2015.

In full, the report:

Looks at how retailers are shifting their ad spending and marketing efforts to keep up with online retail behavior

Identifies which channels are top performers for referral traffic and new opportunities for reaching consumers

Analyzes how retailers are responding to the rise of mobile purchasing and where they're falling short

Examines the evolving delivery landscape and the aggressive moves retailers are making to become their own shipping carriers

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